1. Field of the Invention
This invention relates to a power source apparatus provided with a driving battery for powering the motor of an electric vehicle, such as a hybrid car or electric automobile, or a power source apparatus for an electrical appliance that drives a load, and in particular, relates to a power source apparatus provided with a temperature detection circuit to detect battery temperature.
2. Description of the Related Art
A driving battery, which supplies power to an electric motor that propels a car, has many rechargeable battery cells connected in series to increase output voltage. This is because the electric motor must have high output to propel a car. In this power source apparatus, battery cell temperature becomes high during high current discharge for rapid car acceleration via the motor, during high current charging near full charge while driving on a long hilly road with regenerative braking, or when outside air temperature becomes abnormally high and battery cell ambient temperature becomes high.
Further, in a car power source apparatus, an extremely large number of battery cells, such as 100 or more, are connected in series to increase output voltage to 200V to 400V, for example. When many individual battery cells are connected in series, and charged and discharged, the same current flows in all the battery cells. However, not all battery cells degrade uniformly. Only a particular single battery cell may degrade. Each battery cell will not degrade in exactly the same fashion due to differences such as manufacturing process variation and external environmental factors like operating temperature. If one battery cell degrades and its actual full charge capacity decreases, it will become over-charged or over-discharged when charged or discharged in the same fashion as all the other battery cells. As a result, temperature of the degraded battery cell will increase and even more degradation will occur.
For example, nickel hydrogen (nickel hydride) batteries are used as battery cells of a driving battery. However, for any rechargeable battery, if temperature increases and electrical characteristics degrade, thermal run-away and rapid battery degradation can result from further temperature increase. To avoid this hazard, the car power source apparatus detects the temperature of each battery cell. If the temperature of any battery cell rises, for example, above 80° C., it is judged as the onset of thermal run-away, and contactors (power relays) connected at the output-side of the driving battery are switched OFF to cut-off current flow and prevent damage due to rising temperature.
To realize this high temperature cut-off, PTC temperature sensors were attached in contact with each battery cell in prior art power source apparatus (refer to Japanese Patent Application Disclosure HEI 10-270094 (1998)). Many PTC devices, which were attached to each battery cell, were connected in series and in-turn connected to the input-side of a temperature detection circuit. When temperature increases above the set temperature of a PTC device, its electrical resistance rapidly increases. Therefore, by detecting the total electrical resistance of many PTC devices connected in series, a temperature greater than the PTC set temperature at any battery cell could be detected. This is because the resistance of the series connected PTC devices will increase when the temperature of a battery cell exceeds the set temperature.
However, a temperature detection circuit that detects the electrical resistance of many series connected PTC devices cannot accurately detect the temperature of an individual battery cell. This is because it is difficult to assemble PTC devices all with uniform temperature characteristics. For example, a circuit to detect the temperature of each battery cell using many PTC devices with 10% error in the set temperature (for rapid resistance increase) cannot accurately detect an over-set temperature condition for all battery cells. Further, for a temperature detection circuit that detects total electrical resistance of a large number of PTC devices in series, the greater the number of PTC devices connected in series, the more difficult accurate detection of battery cell temperature becomes. This is because even if a PTC device has resistance increase, its resistance increase becomes a smaller ratio of the total resistance of many PTC devices.
With the object of resolving these types of prior art problems; the present inventor developed a power source apparatus having the circuit configuration shown in FIG. 1 (Japanese Patent Application 2004-77318). This power source apparatus is provided with batteries 91 that drive a load, and temperature detection circuits 92 that detect the temperature of the batteries 91. The batteries 91 are made up of a plurality of battery cells 93 connected in series or parallel. Temperature detection circuits 92 are provided with a plurality of temperature sensors 94 disposed in thermal contact with battery cells 93 and having electrical resistance that decreases when battery temperature becomes high; series resistors 95 connected at one end to a power supply 99 and at the other end to connection nodes 96 for each temperature sensor 94 and causing connection node 96 voltage to change with temperature sensor 94 resistance; and first diodes 98 connected at the connection node 96 of each temperature sensor 94. The polarity of the first diode 98 is such that it conducts in a direction towards the connection node 96. In this power source apparatus, if the temperature of any battery becomes greater than the set temperature, electrical resistance of the temperature sensor 94 in thermal contact with that battery drops and the voltage of the connection node 96 decreases. Consequently, if the voltage of a connection node 96 drops below a set value, current is cut-off to prevent thermal run-away.
The power source apparatus of FIG. 1 can detect battery temperature with a high degree of accuracy and prevent battery thermal run-away. Further, there is no loss in the temperature detection accuracy for each battery even as the number of temperature sensors is increased. However, this power source apparatus requires a dedicated circuit to prevent battery thermal run-away. As a result, it has the drawback of high cost for circuitry to prevent battery thermal run-away.
The present invention was developed to further resolve this drawback. Thus, it is an important object of the present invention to provide a car power source apparatus that can reliably prevent battery thermal run-away with an extremely simple circuit structure.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.